This invention relates to new and improved centrifugal cleaners and separators. For purpose of convenience, it is to be understood that any reference herein to cleaners shall be considered to likewise include a reference to separators.
The present invention was developed in the process of trying to overcome problems evidenced in the use of prior art centrifugal cleaners in the pulp and paper industry stemming from the adoption of the "whole tree" chip concept. This concept resulted from the need to utilize as much of a tree as possible in preparing pulp, due to diminishing resources, and, more importantly, to reduce the number of operations, the investment in capital equipment and thereby the cost of preparing raw material for conversion to pulp.
Since adoption of the "whole tree" chip concept and its application in pulping procedures, pulp slurry delivered to centrifugal cleaners for separation of good fiber materials will in many cases include significantly increased quantities of bark and the underlying cambium layer the character of which is unlike normal shives and fibers. This material tends to interfere with the normal operation of the cleaner and reduces its efficiency. Moreover, the nature and quantity of the cambium layer involved is often such that it has been found difficult to remove it from a slurry, particularly in a single pass through a centrifugal cleaner. The bark particles also carry embedded impurities. This last raises the normal percentage of what might be called "rejects" entrained in the slurry.
The foregoing problem has been compounded by governmental requirements to close water systems in pulp and paper mills so discharge of pollutants may be better controlled. This has resulted in an increase in temperature of the liquid employed, which is caused thereby to have a lower viscosity, undesirably affecting the cleaning or separating ability of the centrifugal cleaner. What happens within a cleaner in such case is that as slurry moves through the cleaner in a vortex type flow pattern, producing counterflowing vortices, the liquid tends to release from the entrained solids and move to the inner vortex in the cleaner much more quickly than it would under usual temperature conditions. The entrained solids are left adjacent the cleaner wall and by reason of their thickened and matted condition they tend to trap individual fibers which otherwise would normally have been carried into the center of the cleaner for flow to and through its accepts nozzle. This abnormal behavior of the vortex flow produces intermingling of dirt and fibers in a manner that fibers will oftentimes carry dirt to the inner vortex, consequently increasing the objectionable solids reaching the accepts nozzle. At the same time a good portion of the fibers which should flow through the accepts nozzle remain entangled with the solids directed to the rejects or underflow nozzle of the cleaner. Limiting rejects does not help the situation as one would normally expect since this can cause plugging of the rejects nozzle and failure of the cleaner system to operate as intended.
The solution of the foregoing problems was the target of the efforts which resulted in the present invention.